A biaxially oriented polyamide resin film comprising nylon as a main component is strong and superior in gas barrier property, resistance to pinhole, transparency and printability, and widely used as packing materials for various kinds of foods such as liquid food, water-containing food, frozen food, retort-packed food, paste food, livestock meat food and marine food. In recent years, such film has been widely used as a packing material for retort-packed food.
The polyamide resin film used for such packing uses is generally printed and laminated with, for example, a film made from polyolefin resin such as polyethylene and polypropylene, and the obtained laminate is folded in two in parallel with a travel direction, heat-sealed at three sides and cut out to produce a three-sided bag with one open end, wherein the polyamide resin film forms a surface layer. The bag is charged with the above-mentioned food etc., sealed, heat-sterilized in boiling water and put on the market.
The conventional polyamide resin film clearly poses the following problems in terms of processability and quality. That is, the film softens by absorption of humidity under high humidity environments to show degraded slip property, because of which the film is subject to difficulties caused by insufficient slip property during handling and processing, particularly in the rainy season. With respect to the bag applied with a boiling water treatment, moreover, the bag is associated with the problem in that the four sides thereof are curled into S-shapes after heat-sterilization, thus markedly impairing the appearance of a packed product. There has heretofore existed no polyamide resin film which fully satisfies the above-mentioned requirements in properties.
For an improved slip characteristics of the polyamide resin film, there have been proposed the following methods.
(1) A method comprising adding fine particles such as silica and kaolin to a resin, allowing these fine particles to protrude out on the film surface upon drawing to form fine protrusions, thereby reducing the contact area between the films. PA0 (2) A method comprising reducing the interaction of the contact parts of the films by adding, to a polyamide resin, an organic lubricant such as a bisamide compound of a higher fatty acid. PA0 (3) A method comprising preparing the film into a multiple layer structure by coating, laminating, coextrusion and the like and forming, on the outermost layer, a layer improved in slip characteristics which is obtained by the above method (1) or (2).
It should be noted that the above method (1) requires a large amount of fine particles to form many protrusions on the surface of the film so as to secure satisfactory workability under high humidity conditions. Along therewith, the transparency of the film becomes low to become a fatal defect for packing uses which require beautiful appearance. The method (2) can improve slip characteristics with less decrease in transparency as compared with the method (1), whereas an increase in the amount of organic lubricant for achieving sufficient slip characteristics can lead to poor adhesion to other materials and wettability when laminating the film with other materials, thus possibly exerting adverse influence on processing such as printing, evaporation and lamination.
For resolving the problem of curling after heat-sterilization, for example, methods comprising reducing the curling phenomenon by specifying the properties of the polyamide resin film have been proposed as disclosed in Japanese Patent Unexamined Publication Nos. 4-103335 and 4-128027. However, such methods are associated with the following difficulties and hardly considered satisfactory to fulfill the need of the consumer. That is, the method disclosed in Japanese Patent Unexamined Publication No. 4-103335 intends to reduce the above-mentioned curling phenomenon by specifying the evaluation standard which is the product of boiling water shrinkage distortion proportion of the biaxially oriented polyamide resin film and rate of change in molecular orientation angle in the film width direction which is determined using microwave. While this method is successful to some degree in improving the curling phenomenon, it is insufficient with respect to the toughness and resistance to pinhole, thus leaving a room for an improvement including these aspects as well. Japanese Patent Unexamined Publication No. 4-128027 proposes suppressing the curling phenomenon by enhancing the heat resistance of the biaxially oriented polyamide resin film, specifically by decreasing the boiling water shrinkage proportion in all directions of the film to not more than about 3%. In order to increase the dimensional stability during boiling water treatment by this method, it is necessary to promote crystallization of the film materials by extremely elevating the temperature of heat-setting, or relax the tension of the molecular chains of the film-constituting polymer by excessively applying relaxation heat treatment after drawing. As a result, another problem in that the obtained film loses toughness and resistance to pinhole arises.
To sum, the conventional methods for improving curling phenomenon, which are applicable to biaxially oriented polyamide resin film impair, along with suppression of the curling phenomenon, other important, required properties, such as toughness and resistance to pinhole of the film.